"It is convenient to judge molecular events using our natural senses," Sheng-Hsien Chiu told nanotechweb.org. "Although we can use spectroscopic methods to determine the operation of acid/base-controllable molecular switches if they are to be applied, for example, in displays it will be necessary to detect their states visually."
Chiu and colleagues used a complex made up of a pH-controllable molecular switch and a molecular clip with TTF side walls. The molecular switch consisted of a [2]rotaxane with a dumbbell component that had dibenzylammonium and 4,4'-bipyridinium "stations" that were able to "lock up" the ring-shaped component known as a macrocycle. Under basic conditions the macrocyle moved to the bipyridinium station, displacing the molecular clip and dissociating the switch/clip complex.
"The sequential addition of a base and an acid induces migration of the macrocyclic unit of this [2]rotaxane from the NH2+ station to the bipyridinium unit and back again," explained Chiu. "In turn this controls the availability of the bipyridinium station for complexation with the molecular clip and results in significant colour changes that are clearly visible to the naked eye."
The researchers wanted to use an external host molecule because it avoided the synthesis of a complicated molecular switch and also because the same external host molecule may be able to monitor the molecular motion of other molecular switches with similar free recognition stations. "A clip-shaped host molecule seemed to be a good choice because molecular clips bind to their guests without having to overcome the bulky terminal groups of rotaxane molecules," said Chiu. "In addition, their pre-organized molecular structures result in efficient complexation with complementary aromatic guest units."
Now the team would like to use the technique to monitor the operation of molecular switches that work by electrochemical or photochemical methods. "Another application will be the construction – with controllable aggregation behaviour – of exceedingly complex molecular assemblies," said Chiu. "Because rotaxane/molecular clip complexes provide a new mode of operation of molecular machinery, they may lead to new designs of molecular devices, for example 'supramolecular nanovalves' or 'supramolecular fences' that can preserve or control the flow of important molecules within porous materials."
The researchers reported their work in Chem. Commun..